1. Field of the Invention
This invention generally relates to automobile jacks. More particularly, the present invention concerns fluid-actuated automobile jacks of the type which may be mounted between the rails of an automobile hoist.
2. Description of the Prior Art
As used herein, the term vehicle hoist will refer to an apparatus to raise the whole of a vehicle from the ground. One type of vehicle hoist comprises a pair of parallel rails onto which the vehicle drives. The rails are then raised from the ground, thus elevating the vehicle. This type of vehicle hoist is commonly referred to as a "drive-on hoist" and typically is used in aligning vehicle wheels.
During alignment operations, it is often necessary to raise one end of the vehicle from the hoist so that the tires are lifted from the rails. Various means have been used to accomplish this. Most simply, a single-acting piston jack may be placed on one of the hoist rails and used to raise one wheel of the vehicle in much the same manner as when a motorist changes a tire. Vehicle jacks have also been adapted to mount between the rails of the vehicle hoist and, when properly positioned, to raise the entire front end or the entire rear end of the vehicle. Jacks of this generic type are disclosed in the patents to Mueller et al. U.S. Pat. No. 3,556,481 and to Hunter U.S. Pat. No. 4,050,545. Generally, these jacks display a frame which is suspended between the rails of the vehicle hoist, an upper platform with contact shoes for engaging the vehicle frame, hydraulic means for raising the upper platform relative to the frame, and locking means for preventing collapse of the jack in the event of failure of the hydraulic lifting system.
A shortcoming has been noted in the prior art. While the locking systems disclosed will prevent collapse of the jack in the event of hydraulic system failure, the locking systems may be released at any time. Should the locking system be accidentally released while pressure is maintained in the hydraulic system, there is no problem since the load will still be supported. However, should the locking system be accidentally released when there is insufficient pressure in the hydraulic system, the jack will collapse with possible injury to people and property.